Mutations in presenilin-1 (PSEN1) cause autosomal dominant Alzheimer's disease and mutations in MAPT cause the familial tauopathy Frontotemporal dementia linked to chromosome 17 (FTDP-17). However, there have been reports of mutations in PSEN1 and MAPT associated with cases of FTD with ubiquitin-positive tau-negative inclusion pathology. Here, we demonstrate that the MAPT variants are almost certainly rare benign polymorphisms as all of these cases harbour mutations in Progranulin (PGRN). Mutations in PGRN were recently shown to cause ubiquitin-positive FTDP-17.